Four types of fault detection mechanisms usually occur in a typical high-voltage, multi-channel light emitting diode (LED) driver integrated circuit (IC). Referring to FIG. 1, they are LED open 10, LED short 20, over voltage protection (OVP) 30, and over temperature protection (OTP) 40. Each of these faults will usually cause shut down of one or more LED channels, or even the entire IC, to prevent damage to the device.
In a large LCD panel (e.g., at least 10″) using white LEDs (WLEDs) as backlight, each channel typically uses a string of 8-10 WLEDs connected in series. A common boost converter drives the input to the LED strings. Each channel has an independent current source to regulate the current in each of the LED strings. To minimize on-chip power dissipation, the boost output voltage from the LED driver IC is regulated to minimize voltage drop across the current sources. Because of LED forward voltage process tolerance, the voltage drop across each LED string can differ widely. The IC sets the boost output voltage based on the voltage requirement of the LED string with the largest voltage drop.
When a LED channel has an OPEN fault (e.g., OVP=1), the IC will see a large voltage drop across that channel, hence causing the boost voltage to increase. At some point, the boost voltage will hit an OVP limit set by external resistors. Then, the LED-open detector will activate (12) to detect and disable the open channel (14). Thereafter, boost voltage regulation will be controlled by (e.g., using feedback from) the channel with the next largest LED forward voltage drop, and the boost voltage will return to its nominal level.
It is important to select the OVP threshold voltage precisely so as not to cause undesirable device operation. Choosing an OVP threshold voltage that is too low may cause false LED-open detection or device shutdown (32) due to a prolonged OVP condition (OVP_timer_exp=1). On the other hand, choosing an OVP threshold voltage that is too high may cause device overheating and eventual OTP shutdown (42) of the device when a LED channel is suddenly opened. This is because of an increased power dissipation in the other LED channels as the boost voltage is regulated upwards due to the open channel. If the OVP limit is set too high, the die temperature may increase beyond the OTP limit (OTP_timer_exp=1) before OVP is reached (OVP_timer_exp=1), and the LED open channel can be disabled (14). In each case, the entire LCD panel will be inoperable.
Another LED fault condition is when a particular channel has one or more LEDs shorted in the string (20), causing the net voltage drop across that string to be reduced, resulting in higher power dissipation in that channel. A conventional IC will detect this (22) as a fault condition and disable the LED shorted channel (24) to prevent the IC from overheating. When a particular channel is disabled, the LCD panel will likely have a dark patch or a streak in the screen.
OVP and OTP fault conditions are usually non-recoverable and require the entire device to be power-cycled before it can operate again. Therefore, it is very important to pre-select an appropriate OVP threshold based on the worst-case LED string forward voltage tolerance, the worst-case thermal characteristics of the printed circuit board (PCB) containing the driver IC, and the worst-case ambient conditions and operating environment. Due to the a priori nature of the OVP selection process, a lot of guard-banding is typically involved, resulting in a more stringent LED string forward voltage mismatch specification value, which consequently results in higher panel costs.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.